An appliance, a machine or an engine generally consists of different components, which can be regrouped in systems and sub-subsystems. A sub-system is a set of components placed in series and/or in parallel. A system is a set of sub-systems and components placed in series and/or in parallel.
The different systems, sub-systems and components of an appliance, a machine or an engine are designed and dimensioned in order to reach a predetermined service life. This nominal service life can e.g. correspond to the service life of the appliance or machine or to a predetermined maintenance interval of the machine or engine. In order to reach the nominal service life, the system, sub-system or component must be designed in order to be able to withstand the accumulated stress to which it is subject during the nominal service life under nominal operation conditions. Nominal operating conditions can include e.g. a nominal applied load, nominal ambient conditions, etc.
However, the actual operating conditions of the appliance often considerably differ from the nominal operating conditions. In the case of actual operating conditions, which are more severe than the nominal conditions, the system, sub-system or component is subject to an increased stress compared to the nominal stress. This increased stress accelerates the aging of the system, sub-system or component and thus leads to premature failing of the system, sub-system or component and accordingly to increased maintenance costs for the appliance, machine or engine.
In order to avoid premature failing of the system, sub-system or component, the system, sub-system or component has to be designed in order to withstand the stress of worst case operating conditions. Such a design increases of course the manufacturing costs of the system, sub-system or component. This means on the other hand that for normal operating conditions, i.e. for operating conditions, which will occur most of the time, the system, sub-system or component will be oversized or over-dimensioned.